Posterior Analytics (Bouchier)/Book II/Chapter XI

Chapter XI: The kinds of Causes used in DemonstrationEdit

To know a thing is to know its cause; and the Causes, each of which may be used as a middle term in demonstration, are (1) The substantial or Formal cause; (2) The necessary conditions of a thing, or Material cause; (3)That which gave the first impulse to a thing, or Efficient cause; (4) That for the sake of which a thing is done, or Final cause.
Necessity is of two kinds: (1) Obedience to natural impulse; (2) Obedience to external compulsion.

We suppose ourselves to have scientific knowledge of a thing when we have learned its cause. The causes are four in number. First, the essential conception of things; Second, the conditions from which the phenomena necessarily follow. Third, that which gave the first impulse to the thing. Fourth, that for the sake of which the thing happens. All these causes may serve as middle terms. The material cause cannot be demonstrated by means of one premise only, there must be at least two, and that can only happen when one middle term is added. When this is done a conclusion must necessarily follow. This may be made clear from the following example; ‘Why is the angle in a semicircle a right angle? or, under what conditions is it a right angle?’

Let A be right angle, B the half of two right angles, C the angle in a semicircle. Now B is the cause of A, right angle, being predicable of C, the angle in a semicircle; for this latter is equal to A, and C is equal to B, for it also is a half of two right angles. As then B is a half of two right angles A is predicable of C, that is the angle in a semicircle is a right angle. This cause is however the same as the formal cause, which gives the definition of ‘right angle.’ Further it has been proved that the formal cause may be used as a middle term.

As another example take the question, ‘for what reason were the Athenians engaged in the Persian war?’ or, ‘What was the cause of the Athenians being attacked?’ ‘Because they assaulted Sardes together with the Eretrians’; for it was that which gave the first impulse. Now let A represent ‘War’; B, ‘making the first assault’; C, ‘the Athenians.’ Here B, making the first assault, is true of C, the Athenians, and A, war, is true of B, for men fight against those who have done the first wrong. Hence A, being attacked, is true of B, those who did the first wrong, and B is true of C, the Athenians, for they were the aggressors. Hence in this case also the middle term is a cause, namely the efficient cause. As an instance of the final cause take the question: ‘For what reason is he walking?’ ‘In order that he may keep well.’ ‘What is the object of a house?’ ‘The preservation of furniture.’ Thus, the purpose of the former is ‘keeping well,’ of the latter ‘preserving furniture.’ [There is no difference between the cause which makes him walk after supper and the final cause of his walking]. Let C represent ‘a walk after supper,’ B ‘food not remaining undigested,’ A ‘keeping well.’ Let it be assumed as an attribute of walking after supper that it prevents food remaining undigested at the entrance to the stomach, and that the absence of this latter produces health. Now food not remaining undigested is considered to be an attribute of C, walking; and A, health, of B. What then is the reason why A, the final cause, is an attribute of C? Clearly it is B (food not remaining undigested), and B is in a manner the cause of A, for it is through it that A will be explained.

This may also be expressed as follows: ‘Why is B an attribute of A?’ ‘Because being in such a condition as that denoted by B constitutes keeping well.’ The matter would be made still more clear if we substituted the notion of final cause for that of efficient cause in this example.[1] The origins of a thing will appear in an inverted order in connection with the efficient cause to that in which they appear in the final cause syllogism, for in the former case the middle term or efficient cause must precede the phenomenon, while in the case of the final cause the minor term C is in point of time earlier, the final cause itself (A) coming last in time.

It is possible for a thing to have a final cause and yet to be necessary; e.g. Why does light pass through a lantern? Because that which consists of the smallest parts necessarily passes through the larger apertures. Thus light is produced because it passes through the lantern in this particular way, and it also has a final cause—namely to prevent us from stumbling.

As then a thing which has a final cause and is necessary can exist, so also such a thing can come into existence. For instance, suppose thunder to arise both because there must necessarily be hissings and roarings when fire is quenched and also, as the Pythagoreans hold, in order to menace those in Tartarus and inspire them with dread. Most instances are of this kind, especially things which are or have been produced by natural laws, for nature works in some cases with a definite purpose, in others of necessity.

Necessity itself has two aspects, one kind being that which obeys nature or a natural impulse, another that which acts under compulsion and contrary to its own impulse. Thus a stone moves both upwards and downwards ‘of necessity,’ though not owing to the same kind of necessary. With regard to the results of intellectual processes some things are never produced by accident, but with some end in view (as House or Statue), others from chance (as Health, or Deliverance from danger). The final cause is of the greatest importance in the case of contingent matters, when the origin of the phenomenon is not fortuitous, and the object aimed at, whether natural or artificial, is something good. Nothing however which comes about by chance can have any definite object.


  1. I.e. the efficient cause is demonstrated by means of the final cause. In full the syllogism would be:—A (keeping well, the final cause) accompanies good digestion (B). But C (the efficient cause) produces A; therefore C produces B.